In underwater repair operations, fillet welds are used extensively due to the relative ease of installing of a fillet weld as opposed to installing a butt weld. A butt weld takes more skill on the part of the welder and is more susceptible to defects due to the varying influences encountered in the underwater environment. For example, if a crack is found in an underwater structure such as the hull of a ship, it is generally preferred to weld a "doubler" or additional plate over the cracked area using a fillet weld than it is to cut out the hull plate around the defective area and install a sound piece of plate in its place. Although the butt weld will give a smoother contour to the hull, the ship must be dry-docked during the operation or else a watertight habitat must be built around the defective area. Moreover, the defective material that is cut out must be discarded without damaging the habitat and a new piece of plate must be cut to size to exactly fit the hole where the damaged plate was removed and the diver must have access into and out of the habitat without disturbing the watertight environment. The new plate must be carefully held in alignment during the initial stages of welding and the welder has to weld into an "open groove" with poor edge preparation and no backing. Any water, oil, grease or other bilge on the interior of the ship must be drained and cleaned to bare the metal in the welding area.
In constrast, a double plate type of repair may be performed in the wet, that is, underwater, with minimum cleaning and edge preparation, is uncomplicated and requires only minor skill on the part of the welder. Thus, fillet welding doubler plates is eminently more practical in terms of expense, time and quality of the end result and so is used in almost all repair operations performed underwater.
Devices for ultrasonic inspection have been used extensively on land and, to some extent, underwater in determining the quality of butt welds; however, ultrasonic inspection of "fillet" or "lap" welds has not been effectively developed. The reason for this is the inherent geometry of the lap joint which does not lend itself to the current methods of straight beam or shear wave inspection, and fillet welds generally are used on less critical joints where inspection has not been required. Recently, however, because of more stringent quality control of repair operations, the need for a device for providing for better inspection of fillet welds has risen, since that used for butt welds is not suitable. It is the purpose of this invention to provide inspection means designed particularly to enable accurate determination of the conditions of fillet welds, including such defects as lack of penetration, lack of fusion, slag inclusions, porosity, cracks, intergranular cracking of the weld and base metal, corrosion, stress corrosion cracks, inclusions and other defects.